You can add any images you wish to the library. There are many free and paid sets available on the internet.

Place your "custom" images in a folder and then use File - Options - Package Folders - New to "link" to that folder. They will then appear in the images library.

When images are added to a map they are copied to the map file. The larger your images, the larger your map file will get. It may be judicious to re-size images in your custom folder to suit their purpose. You only need hi-res large images when you use photographs which are going to be printed in large formats.